docs: def facets_by_vertex(self):
          """
          Returns a list of non-redundant halfspace indices for each vertex
          e.g: facets_by_vertex[0] is the list of indices of halfspaces
          incident to vertex 0
          """
          if self._fbv_out is None:
              output = qhalf('Fv', self.halfspaces, self.interior_point)
              facets = []
              for l in output[1:]:
                  facets.append([int(i) for i in l.split()[1:]])
              self._fbv_out = facets
          return self._fbv_out